Estimation of formation properties is crucial to successful reservoir production and management. Reservoir simulators typically divide the reservoir into grid blocks and use finite difference techniques to model fluid flow in the reservoir. They require formation property and initial condition input for each grid cell. However, formation properties estimated from log data are valid only in the near wellbore region on the order of a few feet and are a one-time measurement immediately after drilling. To populate the simulator grid blocks in the regions between wells, these local properties are interpolated. For heterogeneous reservoirs, such interpolation using sparse local data leads to erroneous predictions of reservoir performance. In order to predict fluid movement in the reservoir accurately and control it, these formation properties must be known in both the local and reservoir scale. Drilling monitoring wells (such as observation wells, producing wells, exploratory wells, etc.) and instrumenting them with permanent sensors (or sensors in pseudo-permanent temporary wells), in accordance with the present invention, allows data collection over the life of the reservoir. Preferably, the location of one or more wells should be in regions of the reservoir where there is least information available or there is large heterogeneity and uncertainty in formation properties.
Conventional methods for formation characterization use various well logs and core analysis to estimate formation properties such as porosity, density, mineralogy, etc. Resistivity logs that make shallow and deep measurements are used to estimate water saturation. Invasion of mud filtrate into the formation during drilling results in salinity and saturation fronts in the near wellbore region. The separation between the two fronts depends on connate water saturation, irreducible water saturation and maximum residual oil saturation. Further, the velocities of the fronts depend on the above parameters as well as the total flow rate (filtrate loss) and porosity. The existence of these two fronts causes radial resistivity variation in the near wellbore region which is measured by array induction logs. By using the array induction log measurements and log interpreted porosity, Ramakrishnan and Wilkinson were able to invert for the total filtrate loss, local connate water, irreducible water and maximum residual oil saturations as disclosed in “Water Cut and Fractional Flow Logs from Array Induction Measurements” SPE Reservoir Eval. & Eng. 2, 1999, pages 85-94 and commonly owned U.S. Pat. No. 5,497,321 (the '321 Patent) to Ramakrishnan and Wilkinson. From this information, the residual oil saturation and relative permeability curves for the local near wellbore region may be estimated. While the method of the '321 Patent is effective in the near wellbore region, its usefulness is limited in the inter-well region. The Ramakrishnan and Wilkinson article and the '321 Patent are incorporated herein by reference in their entireties.
In recent years, permanent monitoring technology has seen rapid progress as a key to improved reservoir understanding and management. Resistivity arrays permanently installed in producing and observation wells are being used as sensors to detect the arrival of the oil-water saturation front, such as that disclosed in commonly owned U.S. Pat. No. 5,642,051 to Babour et al. (the '051 Patent). Voltages at the array electrodes are monitored continuously and changes in these voltages are interpreted as saturation changes due to oil-water front movement tens of feet into the formation. Permanently installed pressure gauges that are hydraulically isolated from the well but in communication with the formation can track formation pressure as disclosed in commonly owned U.S. Pat. No. 5,467,823 to Babour et al. (the '823 Patent). The '051 and '823 Patents are incorporated herein by reference in their entireties.
Commonly owned U.S. Pat. No. 6,182,013 (the '013 Patent) to Malinverno et al. relates to a method and apparatus to dynamically map the location of an oil-water saturation front and to predict its movement over time by combining permanent resistivity array and pressure measurements obtained from apparatus and methods described in the '051 and '823 Patents. The '013 Patent discloses the ability to monitor the movement of a saturation front into a formation by characterizing the formation over a finite distance into the formation at one instance in time; it does not disclose the ability to track arrival time for one or more fronts in a monitoring location. The '013 Patent is incorporated by reference herein in its entirety.
It is therefore an object of the present invention to provide a methodology to provide a method and apparatus to monitor the arrival of at least one front (i.e., a saturation front and a salinity front) to assist in the characterization of formation properties in the inter-well region.
It is a further object of the present invention to estimate effective porosity, connate water saturation, residual oil saturation and relative permeability curves for the inter-well region by monitoring the arrival of both oil-water saturation and water salinity fronts by using permanent resistivity arrays in monitoring locations.
It is another object of the present invention to increase the robustness of the interpretation by using a permanent pressure gauge and/or permanent geophones (time-lapse seismic) together with a permanent resistivity array to independently track arrival/movement of oil-water saturation front.
It is yet another object of the present invention to increase the inversion robustness by independent tracking of the salinity front using a permanent salinity sensor (i.e., dielectric or surface salinity sensors) at the monitoring location.